Ti-rich garnets: an EPMA, SIMS, Mossbauer, XRPD and SCXRD investigations

A suite of Ti-bearing garnets from magmatic, metamorphic and carbonatitic rocks was studied by Electron Probe Microanalysis (EPMA), X-ray Powder Diffraction (XRPD), Single Crystal X-ray Diffraction (SCXRD), Mössbauer spectroscopy and Secondary Ion Mass Spectrometry (SIMS) in order to better characterize their crystal chemistry. The studied garnets show TiO2 varying in the ranges 4.9(1)-17.1(2) wt.% and variable Fe3+/ΣFe content. SIMS analyses allowed quantification of light elements yielding H2O in the range 0.091(7)-0.46(4), F in the range 0.004(1)-0.040(4) and Li2O in the range 0.0038(2)-0.014(2) wt%. Mössbauer analysis provided spectra with different complexity, which could be fitted to a number of components variable from one (YFe3+) to four (YFe2+, ZFe2+, YFe3+, ZFe3+). A good correlation was found between the Fe3+/ΣFe resulting from the Mössbauer analysis and that derived from the Flank method (Höfer & Brey, 2007). X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with very close cubic unit cell parameters as recently found by other authors (Antao, 2013). Single crystal X-ray refinements using anisotropic displacement parameters were performed in the Ia-3d space group and converged to R1 in the range 1.63-2.06 % and wR2 in the range 1.44-2.21 %. Unit cell parameters vary between 12.0641(1) and 12.1447(1) Å, reflecting different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are: YR4+ + ZR3+ ↔ ZSi + YR3+ (schorlomite substitution); YR2+ + ZR4+ ↔ 2YR3+ (morimotoite substitution); YFe3+↔ YR3+ (andradite substitution) with ZR4+ = Ti; YR4+ = Ti, Zr; YR3+ = Fe3+, Al3+, Cr3+; ZR3+ = Fe3+, Al3+ and YR2+ = Fe2+, Mg2+, Mn2+. The 2YTi4++ ZFe2+ ↔ 2YFe3+ + ZSi4+, the hydrogarnet substitution [(SiO4)4-↔ (O4H4)4-], the F– ↔ OH– and the YR4+ + XR+ ↔ YR3+ + XCa2+, with YR4+ = Ti, Zr; YR3+ = Fe3+, Al3+, Cr3+; XR+ = Na, Li also occur. The garnet crystal chemistry and implications in terms of nomenclature and classification (Grew et al., 2013) are discussed. Antao S.M. 2013. The mystery of birefringent garnet: is the symmetry lower than cubic?. Powder diffr., 28(4), 281-287. Grew E.S., Locock A.J., Mills S.J., Galuskina I.O., Galuskina E.V. & Hålenius U. 2013. Nomenclature of the Garnet Supergroup. Am. Mineral., 98, 785-811. Höfer H.E. & Brey G.P. 2007. The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis. Am. Mineral., 92, 873-885.